This invention relates to a moisture-curing polyurethane prepolymer of oleochemical polyols and polyisocyanates. The invention also relates to the use of this prepolymer in moisture-curing adhesives and foams, more particularly in foams produced from non-reusable pressurized containers using blowing gases liquefied under pressure.
Polyurethane prepolymers of the type in question are known. Thus, in EP 125 579, the oleochemical polyols are obtained from epoxidized triglycerides by reaction with low molecular weight monoalcohols, for example by reaction of epoxidized soybean oil with methanol or ethanol. This ring-opening reaction is accelerated by acidic catalysts which, on completion of the reaction, are neutralized with a basic substance, preferably with an amine. Together with polyisocyanates, these oleochemical polyols form NCO prepolymers with a considerable, but not always adequate storage stability when known tertiary amines are added as accelerators for the reaction of the NCO groups with water.
Corresponding polyurethane prepolymers are described in DE 36 26 223. The polyols used in this document are preferably ring opening products of epoxidized oils with monoalcohols. In this case, however, the epoxide groups are completely reacted. In addition, the triglyceride obtained, which contains secondary OH groups and OR groups, is additionally reacted with excess monoalcohol. A mixture of partial glycerides with unreacted triglyceride is formed in this transesterification reaction. These polyols are also produced in the presence of acid catalysts. Finally, the catalyst is again neutralized with an amine. These oleochemical polyols also form polyurethane prepolymers with inadequate stability in storage, particularly when the usual catalysts are present in relatively high concentrations for the moisture curing process.
Polyether polyols obtained by addition of alkylene oxides to low molecular weight diols or triols in the presence of alkaline compounds also form moisture-curing polyurethane prepolymers with limited stability in storage after reaction with polyisocyanates. This applies, for example, to polyols of propylene oxide and/or ethylene oxide and glycerol or trimethylol propane in the presence of NaOH, KOH, MeONa and MeOK in concentrations of more than 10 ppm metal content. Accordingly, to obtain satisfactory storage stability with polyols such as these, the alkali metal content of the polyols is reduced by additional purification measures to concentrations of about 5 ppm Na or K.
Accordingly, there is a need for polyurethane polymers having improved stability in storage for otherwise the same production and performance properties.
The solution provided by the invention is defined in the claims and consists essentially in the use of basic lithium compounds in the production of polyols, more particularly oleochemical polyester polyols.
Accordingly, the polyurethane prepolymer containing NCO groups according to the invention of oleochemical polyols and polyisocyanates is characterized in that it contains up to 14 ppm of lithium and 0.1 to 2.0% by weight of a tertiary amine to accelerate the moisture curing process and is free from Na, K and other amines, particularly salts thereof with strong acids.
N-substituted morpholines above all are suitable for accelerating the reaction of water with NCO groups. Specific examples include 2,2xe2x80x2-dimorpholinodiethyl ether, N-ethyl morpholine, 2,2-bis-(dimethylaminodiethyl)-ether or mixtures of the compounds mentioned. 2,2xe2x80x2-dimorpholinodiethyl ether (DMDEE) is preferably used. These catalysts are particularly suitable because they catalyze above all the curing reaction and, to a lesser extent, the polymerization of the NCO groups (trimerization) or the urethanization reaction. The catalysts are used in a concentration of 0.1 to 2.0% by weight, preferably in a concentration of 0.2 to 1.2% by weight and more preferably in a concentration of 0.5 to 1.0% by weight, based on the prepolymer as a whole. They are generally added to the polyol or to the polyisocyanate before the prepolymerization.
The lithium is used as a basic lithium compound. Basic lithium compounds in the context of the invention are understood to be the hydroxides and alcoholates. However, compounds, such as lithium acetate, lithium oxides and Li metal, may also be used. They are employed in a concentration of 0.1 to 4.0 and preferably 0.5 to 2.0 mmole/kg polyol. This corresponds to a concentration of 0.35 to 14 and preferably 1.75 to 7.0 ppm of lithium in the polyurethane prepolymer. The lithium compound may be added to the reaction mixture as a basic transesterification catalyst either before or during the transesterification reaction. However, where the transesterification is acid-catalyzed, the acid may be neutralized with the lithium compound.
The oleochemical polyols preferably used are polyols containing ester groups which are derived from natural fats and oils. Accordingly, they contain structural elements of fatty acids and fatty alcohols. Starting materials for the oleochemical polyols of the polyurethane prepolymers according to the invention are fats and/or oils of vegetable and/or animal origin with preferably unsaturated fatty acid residues. Specific examples are castor oil, rapeseed oil and soybean oil. However, beef tallow, palm oil, peanut oil, sunflower oil and fish oil may also be used. These starting fats are preferably epoxidized and transesterified. Of the epoxidized triglycerides, epoxidized soybean oil is preferably used. The epoxidation and the reaction with alcohols or carboxylic acids is known (see, for example, EP 125 579 and DE 36 26 223). The polyols formed contain ether groups besides the ester groups.
To produce the polyester polyols according to the invention, polyhydric alcohols with a functionality of 2 to 4 are required in addition to the fats and oils. They have a molecular weight below 400. Suitable polyhydric alcohols are, for example, glycol, glycerol, pentaerythritol and trimethylol propane. Besides these polyhydric alcohols, dicarboxylic acids may also be used in small amounts. Specific examples are adipic acid, phthalic acid, isophthalic acid, terephthalic acid, glutaric acid, succinic acid, azelaic acid, dimer fatty acid and mixtures thereof.
The esterification is carried out under conditions known per se.
After the transesterification, the oleochemical polyols to be used in accordance with the invention have an OH value of 100 to 400 and preferably in the range from 150 to 350.
Besides these oleochemical polyols, the polyisocyanates are the second important structural element for the polyurethane prepolymers according to the invention.
Particularly suitable isocyanates are aromatic polyisocyanates based on MDI (methylene-bis-diphenyl isocyanate or diphenylmethane-4,4xe2x80x2-diisocyanate). Mixtures of MDI with relatively high molecular weight homologs having an average isocyanate functionality of 2.3 to 2.8 are particularly suitable. Other aromatic isocyanates include NDI and TDI. Cycloaliphatic isocyanates, such as IPDI, H12-MDI, and aliphatic diisocyanates, such as tetramethylene diisocyanate and hexamethylene diisocyanate, are also suitable.
The oleochemical polyol and the polyisocyanate are used in a quantity ratio which corresponds to a molar ratio of OH to isocyanate groups of 1:3 to 1:11 and, more particularly, 1:4 to 1:6.
The reaction between the oleochemical polyol and the polyisocyanate is normally carried out at a temperature of 0 to 100xc2x0 C. and preferably at a temperature of 15 to 50xc2x0 C. Catalysts for the reaction of the isocyanate groups with the OH groups are generally not necessary. However, catalysts may be added in small quantities to control the reaction, accelerating the reaction of the isocyanate group with the OH group, but not the trimerization thereof. Specific examples are dimorpholinodiethyl ether, bis-(dimethyl-aminoethyl)-ether, Dabco X-DM (Air Products) and N-ethyl morpholine. However, other catalysts may also be used providing they do not trimerize the isocyanate groups in storage, for example N-substituted morpholines and mixtures thereof with propylene oxide adducts of triethanolamine and the known metal catalysts, particularly tin.
The polyurethane prepolymer has an NCO functionality of 2 to 5 and more particularly 2.5 to 4.2, and an NCO content of 8 to 30% by weight and, more particularly, 10 to 23% by weight, based on the prepolymer, and a viscosity of 5 to 200 and, more particularly, 10 to 100 Pas at 25xc2x0 C. (as measured in accordance with DIN 53015).
To produce polyurethane foams, at least one amine catalyst for the reaction of the isocyanate group with the OH groups (including water), at least one blowing agent and at least one foam stabilizer are required in addition to the polyurethane prepolymer. In addition, other additives may be incorporated, including for example solvents, flameproofing agents, plasticizers, cell regulators, emulsifiers, fungicides, fillers, pigments and antiagers. The composition is a solution or emulsion.
1,1,1,2-Tetrafluoroethane, 1,1-difluoroethane and dimethyl ether is preferably used as the blowing agent. However, carbon dioxide, dinitrogen oxide, n-propane, n-butane and isobutane may also be used. Preferred blowing agents and solvents are chlorine-free fluorocarbons with boiling points of xe2x88x9240 to +60xc2x0 C., propane/butane mixtures and dimethyl ether or mixtures thereof.
The foam-forming composition additionally contains stabilizers. xe2x80x9cStabilizersxe2x80x9d in the context of the present invention are understood to be on the one hand stabilizers which stabilize the viscosity of the composition during production, storage and application. Suitable stabilizers of this type are, for example, monofunctional carboxylic acid chlorides, monofunctional highly reactive isocyanates and non-corrosive inorganic acids. Examples include benzoyl chloride, toluene sulfonyl isocyanate, phosphoric acid or phosphorous acid.
On the other hand, stabilizers in the context of the present invention are also antioxidants, UV stabilizers or hydrolysis stabilizers. The choice of these stabilizers is determined on the one hand by the principal components of the composition and, on the other hand, by the application conditions and by the loads which the foam plastic is expected to withstand. If the main chain of the polyurethane prepolymer is made up of polyether units, antioxidantsxe2x80x94optionally in combination with UV stabilizersxe2x80x94are mainly necessary. Examples of suitable antioxidants are the commercially available sterically hindered phenols and/or thioethers and/or substituted benzotriazoles and sterically hindered amines of the HALS (hindered amine light stabilizer) type.
If the main chain of the polyurethane prepolymer consists mainly of polyester units, hydrolysis stabilizers, for example of the carbodiimide type, are preferably used.
Preferred foam stabilizers are siloxane/oxyalkylene copolymers, for example Tegostab B 8404 (Goldschmidt), Dabco DC-190 or Dabco DC-193 (Air Products).
Quantitatively, the foam-forming composition preferably has the following composition:
50 to 90 and preferably 60 to 85% by weight of isocyanate prepolymers,
0.1 to 2.0 and preferably 0.2 to 1.2 of catalysts,
5 to 35 and preferably 10 to 25 of blowing agents and
0.1 to 5.0 and preferably 0.5 to 3.0 of foam stabilizer.
Of the optional additives, the flameproofing agent may be added in a quantity of 5 to 50 and preferably 10 to 30. The other optional additives may be added in a quantity of 0.1 to 3.0 and, more particularly, 0.2 to 1.5. These figures represent percentages by weight, based on the composition as a whole.
The foam-forming compositions enable a one-component foam plastic to be produced from a pressurized container, the foam thus produced curing in the usual way on contact with atmospheric moisture. However, two-component foam plastics may also readily be produced where a polyol is added to the composition in equivalent quantities or slightly less than the equivalent quantity. The polyol used is any of the polyols normally used containing 2 to 6 carbon atoms and 2 or 3 preferably primary OH groups.
The foam plastic thus produced is particularly suitable for insulation, assembly and sealing in the building industry, particularly in situ.
The polyurethane prepolymer according to the invention is also suitable for the production of adhesives. They consist essentially of the polyurethane prepolymer and the amine catalyst. There is no need for blowing agents or foam stabilizers.
The polyurethane prepolymer according to the invention is distinguished by high stability in storage. By stability in storage is meant in particular a stable viscosity at storage temperatures of 20 to 50xc2x0 C. In order to be able to compare the storage stabilities of various systems, the viscosity of the polymers is measured after prepolymerization and after various periods of storage at 20xc2x0 C. (room temperature) and 50xc2x0 C. The viscosity measurement itself is carried out at 25xc2x0 C. with a Brookfield viscosimeter of the RVT type. On account of the different initial viscosities, the percentage increase in the initial viscosity per month of storage (=30 days) rather than the absolute increase in viscosity is used as the characteristic quantity for storage stability. This calculated quantity is referred to as the rise in viscosity. In the case of the described polyurethane prepolymers, it is determined solely in the presence of solvents because the preferred application of such prepolymers is in high-solids systems with solvent contents of 5 to 30%. xe2x80x9cFCKW-113xe2x80x9d is used as the solvent. However, other solvents may also be used providing they contain no NCO-reactive groups, for example dimethoxymethane, methyl acetate, pentane, dimethyl carbonate, etc.
The storage stability of the polyurethane prepolymer is determined not only by the type and production of the oleochemical polyol, but also by other factors, namely the type, reactivity and impurities of the di- or polyisocyanate, the type and concentration of the catalysts added to accelerate the NCO/OH reaction in the polyol/isocyanate addition, the type and concentration of the accelerator added for the NCO/H2O reaction and also by the control of temperature during the prepolymerization. The additions of solvents, flameproofing agents and plasticizers also have an additional, but lesser effect on storage stability.